The invention relates to an analytical device, such as a mass spectrometer, gas chromatograph and liquid chromatograph in which accurate temperature control is required for analysis.
In a Time of Flight (TOF) type mass spectrometer, after a sample is ionized, a voltage is applied to accelerate the ion. The ion flies in a flight tube held in a high vacuum and reaches an electrode. A time of flight of the ion is measured to determine a mass (mass/charge) of the ion. When the flight tube is expanded or contracted due to a temperature change, a flight length of the ion is changed, thereby causing an error in the measured mass number. In a quadrupole type mass spectrometer, when a quadrupole rod is expanded or contracted due to a temperature change, a quadrupole electric field in a space surrounded by the quadrupole rod is changed, thereby changing transporting efficiency of the ion and causing an error in the measured ion intensity. In a gas chromatograph, an analysis is carried out based on a difference in a boiling point of a sample. Therefore, when the temperature is not controlled accurately, it is difficult to analyze with good repeatability.
A PID control has been known as a method of controlling a physical quantity with high accuracy (refer to Patent Document 1 and Non-Patent Document 1). In the PID control for controlling a temperature as an example, an output of a heater (or cooler) is a sum of a term (proportional term: P term) in proportion to an error or difference between a present temperature of a subject to be controlled and a preset temperature; a term (integral term: I term) in proportion to an integral of the error; and a term (derivative term: D term) in proportion to a derivative of the error. Incidentally, when such a sum is employed, each term is often modified with a predetermined weight.
When the monitored temperature of the subject to be controlled is Tmon, the preset temperature is Tset, the error is E, and the output of the heater is M, a block diagram of the temperature control through the PID control is expressed as shown in FIG. 1. In FIG. 1, symbol C represents a transfer function of the PID control, and symbol G represents a transfer function of a system of the subject to be controlled. As shown in FIG. 1, when the monitored temperature Tmon is changed due to an external disturbance, the output M of the heater according to the error E between the monitored temperature Tmon and the preset temperature Tset is controlled.
In order to improve the accuracy of the PID control, there has been known a cascade control wherein the PID control is applied in double. In the cascade control, a single control system signal provides a preset value of another control system, so that a loop of the PID feedback works in double (in cascade).
[Patent Document 1]
Japanese Patent Publication (Kokai) No. 2001-357816
[Non-Patent Document 1]
“BASIC AND APPLICATION OF PID CONTROL” Shigehiko Yamamoto and Naotake Katoh, Asakura Shoten (publisher), Apr. 1, 1997
As described above, the PID control and the cascade PID control are suitable for controlling a physical quantity with high precision. When a temperature is controlled, if a system of a subject to be controlled (subject to temperature control) has large thermal capacity, the system has a large time constant, and a waste time (from start of the control to when a monitored value changes) becomes large. Accordingly, it is difficult to adjust a coefficient or parameter of the PID control. In the analytical device requiring accurate temperature control as described above, it is more difficult to control the temperature. In particular, when the temperature control is started, an overshoot or an excessive long control time may occur. Also, it is difficult to control against an external disturbance such as a rapid change in an outside temperature.
In view of the above problems, the present invention has been made, and an object of the invention is to provide an analytical device with a temperature control device capable of controlling a temperature with high accuracy.
Further objects and advantages of the invention will be apparent from the following description of the invention.